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Salt Lake Astronomical Society

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Stansbury Park Observatory Complex (SPOC)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stansbury Park Observatory Complex (SPOC) clear sky chart from 2005-11-06 to 2025-11-03.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	5% 7% 2 6% 4 4 4 4 5% 7% 53%	135524
	Night	6% 8% 3 6% 4 4 4 4 5% 7% 50%	131987
Transparency	Day	29% 7% 9% 20% 22% 14%	130247
	Night	31% 6% 9% 21% 20% 12%	128267
Seeing	Day	8% 11% 26% 33% 18% 4	16106
	Night	10% 6% 20% 36% 23% 4	26372

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	6% 20% 5% 10% 5% 6% 6% 5% 6% 8% 23%	4098
	11th to 20th	Day	6% 13% 5% 16% 8% 5% 4 5% 5% 8% 25%	4100
	21st to end	Day	6% 12% 5% 8% 5% 4 4 4 4 7% 40%	4510
February	1st to 10th	Day	9% 13% 4 10% 5% 5% 4 4 6% 8% 33%	1800
	11th to 20th	Day	8% 15% 5% 9% 5% 4 5% 4 6% 7% 34%	1855
	21st to end	Day	5% 13% 4 8% 5% 6% 5% 4 6% 6% 36%	1650
March	1st to 10th	Day	4 13% 4 10% 5% 5% 5% 3 5% 7% 40%	4777
	11th to 20th	Day	3 11% 4 8% 6% 5% 5% 5% 6% 7% 41%	5000
	21st to end	Day	6% 14% 4 10% 6% 6% 6% 5% 6% 7% 31%	5486
April	1st to 10th	Day	8% 10% 2 6% 4 5% 5% 4 6% 7% 42%	2200
	11th to 20th	Day	8% 9% 3 7% 5% 4 4 4 6% 6% 45%	2556
	21st to end	Day	7% 7% 2 6% 5% 4 4 4 7% 8% 47%	2600
May	1st to 10th	Day	6% 7% 2 5% 4 4 4 4 6% 7% 50%	5880
	11th to 20th	Day	6% 6% 2 5% 3 4 4 4 6% 8% 51%	5841
	21st to end	Day	4 4 2 5% 3 4 4 3 6% 8% 59%	6423
June	1st to 10th	Day	3 3 1 4 3 4 4 5% 6% 10% 58%	2600
	11th to 20th	Day	4 3 1 4 3 3 3 3 5% 6% 64%	2600
	21st to end	Day	1 2 1 3 2 2 3 3 4 6% 73%	2762
July	1st to 10th	Day	1 2 1 2 2 3 3 3 5% 5% 74%	6011
	11th to 20th	Day	2 3 1 3 3 3 4 3 5% 8% 65%	5880
	21st to end	Day	2 2 1 2 2 2 3 3 5% 7% 71%	6401
August	1st to 10th	Day	1 2 1 2 3 3 3 3 4 6% 71%	5779
	11th to 20th	Day	1 1 1 3 3 3 4 3 5% 6% 70%	5798
	21st to end	Day	3 3 1 5% 4 5% 4 4 5% 6% 62%	6468
September	1st to 10th	Day	2 2 1 3 3 2 2 4 5% 6% 70%	2222
	11th to 20th	Day	3 2 1 3 3 3 4 3 5% 6% 66%	2200
	21st to end	Day	6% 3 1 3 3 2 3 3 4 5% 68%	2200
October	1st to 10th	Day	4 3 1 3 3 3 3 3 4 5% 68%	3473
	11th to 20th	Day	8% 5% 1 3 2 3 3 3 4 6% 61%	3250
	21st to end	Day	7% 6% 2 5% 3 3 3 3 5% 6% 57%	3486
November	1st to 10th	Day	7% 8% 3 5% 4 4 5% 5% 5% 7% 47%	1782
	11th to 20th	Day	10% 8% 3 6% 4 3 4 3 5% 8% 46%	1800
	21st to end	Day	7% 13% 4 7% 6% 5% 4 3 4 6% 41%	1743
December	1st to 10th	Day	5% 7% 2 6% 3 3 4 4 5% 11% 50%	2620
	11th to 20th	Day	5% 12% 4 8% 5% 5% 6% 5% 7% 10% 32%	3163
	21st to end	Day	9% 17% 5% 9% 5% 5% 6% 4 5% 7% 27%	4510
January	1st to 10th	Night	6% 19% 6% 10% 4 4 4 3 5% 7% 32%	5867
	11th to 20th	Night	9% 16% 4 11% 6% 4 5% 4 5% 9% 27%	5880
	21st to end	Night	6% 13% 3 7% 4 3 4 3 5% 6% 46%	6468
February	1st to 10th	Night	8% 12% 3 9% 5% 5% 5% 4 7% 7% 35%	2600
	11th to 20th	Night	7% 14% 4 8% 5% 4 4 3 5% 5% 41%	2429
	21st to end	Night	4 14% 5% 7% 5% 4 4 4 6% 7% 40%	1980
March	1st to 10th	Night	5% 13% 4 10% 4 5% 5% 4 6% 7% 39%	5117
	11th to 20th	Night	4 13% 3 9% 5% 5% 5% 4 7% 7% 38%	5000
	21st to end	Night	5% 11% 4 8% 5% 5% 4 5% 6% 7% 40%	5489
April	1st to 10th	Night	10% 10% 3 4 4 3 3 3 5% 6% 49%	2200
	11th to 20th	Night	8% 9% 2 6% 4 3 4 4 7% 7% 46%	1789
	21st to end	Night	9% 8% 2 6% 4 4 4 4 6% 6% 48%	1800
May	1st to 10th	Night	6% 7% 2 5% 4 4 5% 4 6% 7% 50%	4100
	11th to 20th	Night	5% 4 2 4 3 4 4 3 5% 8% 57%	4074
	21st to end	Night	5% 3 2 5% 4 4 4 4 6% 8% 55%	4474
June	1st to 10th	Night	4 4 2 4 4 3 4 5% 6% 8% 57%	1800
	11th to 20th	Night	4 3 1 4 3 3 3 3 6% 6% 64%	1800
	21st to end	Night	2 2 1 4 3 3 3 3 5% 8% 65%	1623
July	1st to 10th	Night	1 2 1 4 3 3 3 2 4 4 72%	3934
	11th to 20th	Night	2 4 2 5% 4 5% 5% 4 6% 8% 54%	4100
	21st to end	Night	2 3 2 4 3 3 4 3 4 6% 66%	4472
August	1st to 10th	Night	2 2 1 3 3 3 4 3 6% 7% 65%	4033
	11th to 20th	Night	2 3 1 3 3 3 5% 4 5% 5% 66%	4051
	21st to end	Night	4 4 2 5% 4 5% 4 4 5% 5% 59%	4509
September	1st to 10th	Night	4 2 1 4 3 3 4 3 5% 6% 67%	2172
	11th to 20th	Night	4 3 1 4 2 3 3 3 3 6% 67%	2200
	21st to end	Night	6% 2 1 3 3 3 3 2 3 4 70%	2200
October	1st to 10th	Night	4 3 1 3 3 3 3 3 3 4 70%	3722
	11th to 20th	Night	9% 5% 2 4 3 3 4 3 4 5% 59%	3898
	21st to end	Night	6% 8% 2 5% 3 3 4 4 5% 7% 54%	4309
November	1st to 10th	Night	8% 9% 2 6% 3 3 4 3 6% 7% 50%	2561
	11th to 20th	Night	7% 10% 3 6% 4 3 3 4 5% 7% 47%	2600
	21st to end	Night	7% 11% 4 6% 5% 4 3 3 5% 7% 46%	2630
December	1st to 10th	Night	5% 7% 2 5% 3 3 4 3 5% 7% 56%	4570
	11th to 20th	Night	8% 12% 3 6% 4 4 5% 4 6% 8% 41%	5068
	21st to end	Night	9% 14% 5% 10% 5% 4 5% 4 5% 7% 31%	6468

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	50% 10% 17% 15% 7%	4099
	11th to 20th	Day	50% 9% 17% 17% 6% 2	4100
	21st to end	Day	40% 8% 15% 20% 16% 1	4510
February	1st to 10th	Day	49% 7% 13% 19% 10% 2	1800
	11th to 20th	Day	49% 7% 12% 19% 13% 1	1864
	21st to end	Day	47% 6% 12% 21% 12% 2	1650
March	1st to 10th	Day	39% 8% 8% 24% 17% 4	4777
	11th to 20th	Day	37% 10% 11% 29% 13% 1	5000
	21st to end	Day	48% 9% 16% 18% 8% 1	5486
April	1st to 10th	Day	40% 7% 11% 21% 17% 4	2200
	11th to 20th	Day	39% 6% 10% 21% 20% 4	2559
	21st to end	Day	33% 7% 11% 26% 21% 2	2600
May	1st to 10th	Day	27% 10% 9% 24% 23% 7%	5880
	11th to 20th	Day	28% 7% 6% 24% 30% 5%	5844
	21st to end	Day	22% 5% 7% 26% 34% 5%	6424
June	1st to 10th	Day	22% 6% 8% 22% 32% 9%	2600
	11th to 20th	Day	20% 5% 5% 18% 31% 21%	2600
	21st to end	Day	12% 4 3 12% 33% 35%	2762
July	1st to 10th	Day	12% 4 4 7% 17% 57%	5948
	11th to 20th	Day	15% 6% 5% 9% 28% 38%	5811
	21st to end	Day	12% 4 5% 8% 27% 44%	6217
August	1st to 10th	Day	17% 5% 4 18% 28% 28%	4208
	11th to 20th	Day	17% 7% 5% 13% 33% 27%	4197
	21st to end	Day	16% 4 4 13% 32% 31%	4663
September	1st to 10th	Day	15% 6% 5% 17% 30% 27%	2222
	11th to 20th	Day	19% 5% 7% 19% 28% 21%	2200
	21st to end	Day	19% 5% 5% 19% 35% 17%	2200
October	1st to 10th	Day	20% 4 5% 14% 38% 18%	3473
	11th to 20th	Day	24% 5% 5% 28% 30% 8%	3250
	21st to end	Day	26% 6% 10% 32% 24% 3	3486
November	1st to 10th	Day	35% 6% 12% 23% 21% 3	1782
	11th to 20th	Day	36% 5% 11% 32% 15% 1	1800
	21st to end	Day	43% 5% 13% 24% 15% 1	1743
December	1st to 10th	Day	26% 6% 20% 40% 7% 1	2620
	11th to 20th	Day	40% 9% 16% 26% 9%	3162
	21st to end	Day	50% 8% 12% 18% 11%	4510
January	1st to 10th	Night	48% 8% 13% 21% 10%	5867
	11th to 20th	Night	48% 9% 13% 23% 7% 1	5880
	21st to end	Night	35% 8% 15% 23% 18% 1	6468
February	1st to 10th	Night	45% 7% 15% 22% 9% 2	2600
	11th to 20th	Night	43% 6% 11% 23% 17% 2	2430
	21st to end	Night	41% 7% 9% 23% 16% 3	1980
March	1st to 10th	Night	39% 6% 9% 24% 16% 6%	5117
	11th to 20th	Night	38% 9% 11% 25% 16% 1	5000
	21st to end	Night	37% 10% 15% 22% 15% 2	5489
April	1st to 10th	Night	35% 6% 10% 25% 18% 7%	2200
	11th to 20th	Night	34% 7% 9% 23% 24% 4	1789
	21st to end	Night	35% 6% 11% 24% 22% 2	1800
May	1st to 10th	Night	27% 8% 9% 23% 25% 7%	4097
	11th to 20th	Night	21% 7% 6% 26% 34% 5%	4075
	21st to end	Night	22% 6% 8% 20% 38% 5%	4474
June	1st to 10th	Night	24% 6% 7% 23% 29% 11%	1800
	11th to 20th	Night	20% 4 7% 18% 29% 22%	1800
	21st to end	Night	18% 4 3 12% 28% 36%	1623
July	1st to 10th	Night	15% 3 3 7% 13% 59%	3889
	11th to 20th	Night	25% 5% 4 7% 21% 38%	4046
	21st to end	Night	18% 4 5% 7% 21% 45%	4344
August	1st to 10th	Night	18% 6% 6% 12% 28% 29%	2928
	11th to 20th	Night	20% 6% 5% 11% 30% 29%	2928
	21st to end	Night	17% 4 4 10% 24% 41%	3245
September	1st to 10th	Night	19% 4 4 15% 25% 33%	2172
	11th to 20th	Night	20% 4 5% 16% 30% 24%	2200
	21st to end	Night	20% 4 4 16% 31% 25%	2200
October	1st to 10th	Night	18% 4 5% 13% 34% 26%	3722
	11th to 20th	Night	27% 5% 5% 25% 27% 12%	3898
	21st to end	Night	28% 6% 6% 28% 28% 5%	4309
November	1st to 10th	Night	32% 6% 9% 25% 21% 6%	2561
	11th to 20th	Night	34% 5% 11% 29% 17% 3	2600
	21st to end	Night	37% 5% 9% 29% 17% 3	2630
December	1st to 10th	Night	26% 6% 14% 40% 14% 2	4570
	11th to 20th	Night	37% 8% 17% 27% 11%	5068
	21st to end	Night	48% 8% 15% 19% 11%	6468

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 14% 28% 26% 13% 4	300
	11th to 20th	Day	14% 12% 27% 28% 17% 3	300
	21st to end	Day	15% 10% 25% 34% 13% 3	330
February	1st to 10th	Day	17% 17% 29% 25% 9% 3	300
	11th to 20th	Day	13% 17% 32% 24% 12% 1	400
	21st to end	Day	8% 17% 35% 31% 9% 1	414
March	1st to 10th	Day	10% 15% 32% 32% 10% 1	500
	11th to 20th	Day	9% 15% 34% 30% 10% 2	500
	21st to end	Day	16% 17% 34% 28% 5% 1	550
April	1st to 10th	Day	12% 15% 30% 25% 15% 3	500
	11th to 20th	Day	13% 22% 26% 27% 10% 1	492
	21st to end	Day	12% 15% 30% 24% 17% 2	500
May	1st to 10th	Day	7% 13% 38% 28% 11% 3	503
	11th to 20th	Day	8% 13% 28% 28% 18% 5%	528
	21st to end	Day	6% 12% 25% 31% 22% 4	573
June	1st to 10th	Day	3 8% 28% 34% 22% 5%	530
	11th to 20th	Day	5% 14% 32% 30% 16% 3	530
	21st to end	Day	2 9% 27% 39% 18% 4	530
July	1st to 10th	Day	2 4 22% 47% 21% 4	526
	11th to 20th	Day	2 2 22% 46% 22% 5%	524
	21st to end	Day	3 3 15% 46% 26% 6%	575
August	1st to 10th	Day	3 4 23% 43% 23% 4	527
	11th to 20th	Day	3 4 18% 43% 25% 7%	517
	21st to end	Day	5% 5% 18% 37% 28% 6%	583
September	1st to 10th	Day	3 5% 18% 36% 26% 12%	530
	11th to 20th	Day	5% 10% 22% 32% 25% 6%	530
	21st to end	Day	6% 7% 21% 33% 28% 5%	530
October	1st to 10th	Day	7% 9% 26% 30% 21% 6%	455
	11th to 20th	Day	7% 14% 22% 26% 22% 8%	330
	21st to end	Day	11% 9% 25% 33% 20% 2	363
November	1st to 10th	Day	15% 13% 21% 26% 19% 6%	306
	11th to 20th	Day	12% 12% 24% 32% 14% 7%	300
	21st to end	Day	16% 9% 21% 26% 24% 4	300
December	1st to 10th	Day	11% 10% 24% 34% 17% 4	300
	11th to 20th	Day	15% 8% 23% 39% 13% 1	300
	21st to end	Day	20% 12% 25% 28% 13% 1	330
January	1st to 10th	Night	14% 9% 21% 32% 19% 6%	800
	11th to 20th	Night	15% 7% 21% 33% 20% 5%	800
	21st to end	Night	16% 6% 18% 36% 19% 5%	880
February	1st to 10th	Night	17% 5% 23% 36% 15% 4	800
	11th to 20th	Night	14% 5% 24% 37% 17% 3	784
	21st to end	Night	11% 9% 25% 39% 15% 2	660
March	1st to 10th	Night	13% 9% 24% 37% 15% 3	800
	11th to 20th	Night	9% 8% 23% 38% 20% 3	800
	21st to end	Night	13% 8% 29% 36% 12% 2	880
April	1st to 10th	Night	15% 8% 24% 35% 16% 2	800
	11th to 20th	Night	12% 9% 30% 33% 15% 2	598
	21st to end	Night	12% 8% 21% 38% 19% 3	600
May	1st to 10th	Night	9% 6% 25% 40% 18% 2	600
	11th to 20th	Night	8% 5% 20% 35% 28% 5%	591
	21st to end	Night	9% 6% 19% 35% 27% 4	650
June	1st to 10th	Night	6% 6% 16% 39% 30% 4	600
	11th to 20th	Night	4 9% 22% 40% 24% 2	600
	21st to end	Night	3 4 20% 38% 30% 6%	423
July	1st to 10th	Night	4 2 18% 35% 36% 5%	510
	11th to 20th	Night	4 3 15% 46% 30% 2	582
	21st to end	Night	5% 2 14% 31% 44% 4	657
August	1st to 10th	Night	4 2 20% 38% 34% 3	597
	11th to 20th	Night	2 2 18% 41% 33% 4	594
	21st to end	Night	4 4 15% 37% 35% 5%	660
September	1st to 10th	Night	4 5% 13% 35% 35% 8%	777
	11th to 20th	Night	5% 5% 17% 34% 32% 8%	800
	21st to end	Night	6% 4 18% 31% 30% 11%	800
October	1st to 10th	Night	8% 5% 19% 37% 25% 6%	800
	11th to 20th	Night	10% 5% 18% 34% 26% 7%	800
	21st to end	Night	10% 5% 23% 32% 25% 6%	880
November	1st to 10th	Night	13% 8% 17% 32% 24% 6%	788
	11th to 20th	Night	13% 4 22% 38% 16% 7%	800
	21st to end	Night	11% 5% 19% 38% 20% 6%	830
December	1st to 10th	Night	14% 5% 19% 39% 20% 2	1000
	11th to 20th	Night	19% 6% 21% 35% 16% 3	951
	21st to end	Night	19% 9% 24% 32% 15% 1	880

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